The present invention relates to the field of power sources, and in particular to a flyback switching power source whose frequency is responsive to both load level and the input voltage level.
Solid state lighting, and in particular light emitting diodes (LEDs) are rapidly coming into wide use for lighting applications. LEDs with an overall high luminance are useful in a number of applications including backlighting for liquid crystal display (LCD) based monitors and televisions, collectively hereinafter referred to as a matrix display, as well as for general lighting applications.
In a large LCD matrix display, and in large solid state lighting applications, such as street lighting, typically the LEDs are supplied in a plurality of strings of serially connected LEDs, at least in part so that in the event of failure of one string at least some light is still output. The constituent LEDs of each LED string thus share a common current.
LEDs providing high luminance exhibit a range of forward voltage drops, denoted Vf, and their luminance is primarily a function of current. Brightness control of the LEDs may be performed by either pulse width modulation (PWM) or by amplitude modulation. In a PWM brightness control a fixed current is driven through the LED string, and the duty cycle of the fixed current is adjusted in order to control the LED string brightness. In amplitude modulation the amount of current through the LED string is varied directly, thus adjusting the brightness.
The advantage of LED lighting is primarily in its energy efficiency and long life, and as such care must be taken to ensure high efficiency for the power source, while ensuring a high overall power factor. A solid state lighting unit which may be used with any worldwide AC mains input voltage is advantageous, since only a single part need be produced irrespective of market. Thus, the power source is to be arranged to work over a broad range of input voltages. Similarly, the solid state lighting unit is to be arranged to provide high efficiency over a broad dimming range, which in the case of amplitude modulation results in a requirement for high efficiency over a broad range of load currents.